Multiplicity Distributions in DIS at HERA

1. Multiplicity Distributions in DIS at HERA Preliminary Examination Michele Sumstine University of Wisconsin Dec. 19, 2002

2. Outline • Introduction • HERA and ZEUS • Kinematics • Parton Shower & Hadronization Models • Universality of Hadron Production • Data Selection & Simulation of ZEUS Data • Mean Charged Multiplicity vs. Effective Mass • Summary and Plan

3. probe lepton Particle Scattering • Study structure of proton • Scattering via probe exchange • Wavelength • Deep Inelastic Scattering – Q2 large • High energy lepton transfers momentum to a nucleon via probe h : Plank’s Constant Q2: related to momentum of photon Size of proton ~ 1 fm HERA Q2 Range ~ 40,000GeV2 HERA can probe to ~ 0.001 fm

4. Naïve Quark Parton Model • Parton Model • Proton consists of pointlike non interacting constituents (partons) • Quark Parton Model • 3 families of quarks ½ spin particles

5. QCD Theory • QCD Quantum Chromodynamics • Strong force couples to color and is mediated by the gluon • Gluon permits momentum exchange between quarks • Gluons create quarks through pair production • Color confinement: free partons are not observed, only colorless objects -hadrons • Parton distribution function (PDF) gives probability of finding parton with given momentum within proton (experimentally determined)

6. QCD Scale • Running of aS • As scale m increases, aS(m) decreases (m = ET or Q) • Perturbative QCD (pQCD) • Small aS(m) (hard scale) • Series expansion used to calculate observables • Nonperturbative QCD • Large aS(m) (soft scale) • Series not convergent Leading Order (LO) Next to Leading Order (NLO) A = A0 + A1aS + A2aS2 +... HERA DIS Data: Running of aS(m)

7. From Partons to Hadrons hard scattering  parton showers  hadronization • Hard scattering: large scale (short distance) perturbative process • Parton showers: initial QCD radiation of partons from initial partons • Hadronization: colorless hadrons produced from colored partons                          soft process (large distance) - not perturbatively calculable          phenomenological models and experimental input

8. Multiplicity and Energy Flow • The hard scattering process determines the initial distribution of energy • Parton Shower + Hadronization determine the number of charged particles produced • Measure mean number of charged particles produced, (mean charged multiplicity, <nch>), versus the energy available for production of final state hadrons, study the mechanisms of hard scattering, parton showers and hadronization • pQCD & universality of the hadronization process can be tested by comparison with data from e+e- and hadron colliders.

9. HERA Description • 920 GeV p+ (820 GeV before 1999) • 27.5 GeV e- or e+ • 318 GeV cms • Equivalent to a 50 TeV Fixed Target • Instantaneous luminosity max: 1.8 x 1031 cm-2s-1 • 220 bunches • 96 ns crossing time • IP~90mA p • Ie~40mA e+ H1 ZEUS DESY Hamburg, Germany Unique opportunity to study hadron-lepton collisions

10. HERA Data Luminosity upgrade • 5x increase in Luminosity  expect 1 fb-1 by end of 2006

11. ZEUS Detector Electron 27.5GeV • General Purpose Detector • Almost hermetic • Measure ep final state particles: energy, particle type and direction

12. Central Tracking Detector e p • Drift Chamber inside 1.43 T Solenoid • Can resolve up to 500 charged tracks • Average event has ~20-40 charged tracks • Determine interaction vertex of the event • Measure number of charged particles (tracks) View Along Beam Pipe Side View

13. Uranium-Scintillator Calorimeter  = 0.0  = 90.0o  = 1.1  = 36.7o  = -0.75  = 129.1o • alternating uranium and scintillator plates (sandwich calorimeter)  = 3.0  = 5.7o  = -3.0  = 174.3o Positrons 27.5 GeV Protons 820 GeV • compensating - equal signal from hadrons and electromagnetic particles of same energy - e/h = 1 • Energy resolution  e/Ee= 18% / Eh/Eh= 35% / E , E in GeV • Depth of FCAL > RCAL due to Ep > Ee • Used for measuring energy flow of particles. • covers 99.6% of the solid angle

14. ZEUS Trigger 107 Hz Crossing Rate,105 Hz Background Rate, 10 Hz Physics Rate • First Level • Dedicated custom hardware • Pipelined without deadtime • Global and regional energy sums • Isolated m and e+ recognition • Track quality information • Second Level • “Commodity” Transputers • Calorimeter timing cuts • E - pz cuts • Vertex information • Simple physics filters • Third Level • Commodity processor farm • Full event info available • Refined Jet and electron finding • Advanced physics filters

15. e’(k’) Virtuality of exchanged photon e(k)  (q) remnant p(P) Inelasticity: 0  y  1 q’ = Center of mass energy of the ep system Center of mass energy of the *P system Only two independent quantities Kinematic Variables Fraction of proton momentum carried by struck parton 0 x  1

16. Kinematic Reconstruction • Four Measured Quantities: Ee’, e, Eh, h. (p,E) conservation   DIS Event

17. Deep Inelastic Scattering

18. HERA Kinematic Range • Q2 = sxy • 0.1 < Q2 < 20000 GeV2 • 10-6 < x < 0.9 • Equivalent to a • 50 TeV Fixed Target Experiment

19. Modeling Multi-parton Production • Model multiple parton emissions from partons produced in hard scattering • Not possible to perform exact matrix element calculations • Two approaches: Parton Shower and Color Dipole Model Parton Shower Model • successive splitting process • cascade of partons with decreasing virtuality • cascade continues until a cut-off ~ 1 GeV2

20. Color Dipole Model • Chain of independently radiating color dipoles • ep: first dipole between struck quark and proton remnant • gluon induced processes are added in "by hand“

21. Hadronization Models • Use phenomenological models because these processes aren’t calculable in pQCD – low scale • Lund String Model and Cluster Fragmentation Models • Start at low cut-off scale – set of partons from parton shower transformed into colorless hadrons • Local parton-hadron duality • Long distance process involving small momentum transfers • Hadron level flow of momentum and quantum numbers follows parton level • Flavor of quark initiating a jet found in a hadron near jet axis

22. Lund String Fragmentation • color "string" stretched between q and q moving apart • confinement with linearly increasing potential (1GeV/fm) • string breaks to form 2 color singlet strings, and so on., until   only on-mass-shell hadrons.

23. Cluster Fragmentation Model • preconfinement of color (after parton shower) • non-perturbative • splitting after parton shower • color-singlet clusters of • neighboring partons formed • clusters decay into hadrons

24. Similarity of particle production at ee, ep and pp colliders Universality of Hadron Production Can look at just part of the string: assumed to give final state particles proportional to logarithm of mass Aim: Test the universality of hadronization in QCDnch ~ length of the string(s) ~ ln of energy available for hadron production

25. e+e- & ep : Breit Frame Phase space for e+e- annihilation evolves with Q/2 = s/2 Current hemisphere of Breit frame evolves as Q/2 Current region  e+e- annihilation

26. Early Experimental Evidence for Universality • Mean charged multiplicity vs. Q in e+e- and current region of Breit frame at HERA • Linear dependence vs. ln Q observed • Data in e+e- and ep agree universality of hadronization process observed • Also look at <nch> as a function of energy available for particle production (slide to follow) e+e- ep

27. 96-97 Data Sample • Event Selection • Scattered positron found with E > 12 GeV • longitudinal vertex cut: |Zvtx| < 50 cm • scattered positron position cut: |x| > 15 cm or |y| > 15cm (in RCAL) “Box cut” • 40 GeV < E-pz < 60 GeV • Track Selection • Tracks associated with primary vertex • || < 1.75 • pT > 150 MeV • Physics and Kinematic Requirement • Q2 (from double angle) > 12 GeV2 • y(from scattered positron) < 0.95 • y(from hadrons) > 0.04 95842 events before cuts 4798 events after all cuts

28. Event Simulation • Ariadne ’96 v2.0 • Matrix elements at LO pQCD O(s) • Parton showers: CDM • Hadronization: String Model • Proton PDF’s: GRV94 HO parameterization of experimental data* • Q2 > 10 GeV2 • Detector Simulation: software package based on GEANT 19990 events before cuts 7058 events after all cuts *M.Glück, E.Reya and A.Vogt, Phys. Lett. B306 (1993) 391

29. Zeus ’96 Data vs. Ariadne Vertex Position X & Y vertex • MC simulates transverse vertex well Z vertex • Longitudinal vertex is shifted due to partial data Work done by M.SumstineSummer 2002

30. Ariadne ’96-’97 Data Energy & Theta of Scattered Positron Scattered Positron Energy Polar Angle of Scattered Positron Positron energy corrections needed – under study.

31. Position of Scattered Positron at RCAL x position y position Important for reconstruction of Q2 Cut: |x| or |y| > 15cm : Eliminates events close to beam pipe

32. Kinematics: Virtuality Q2 electron method • Energy corrections not yet applied

33. Kinematics: Inelasticity Y electron method Y hadron method • needs electron energy corrections • uses hadronic system variables

34. Tracking: # of Charged Tracks • Number of tracks well described by Ariadne model over two orders of magnitude • Validation of model & CTD tracking simulation

35. Tracking:  & pT First look at tracking, acceptable for use to correct for detector effects

36. Study: <nch> vs. Meff CAL within the CDT acceptance CTD Effective Mass • Hadronic final state within  • Charged part seen as tracks • Energy measured by calorimeter

37. Mean Charged Multiplicity vs. Meff Average number of charged particles vs. effective mass • Reasonable agreement for first look at partial data sample, statistics limited at high Meff • Uncorrected ZEUS ’96 data compared to Ariadne • proportional to ln (Meff)

38. ZEUS Measurement • Investigation of degree of universality in particle production • <nch> consistent with linear dependence vs. Meff • <nch> 15% above corresponding e+e- • Understand color dynamics at the pre- hadronization stage at HERA

39. Summary • First look at Multiplicity Distributions in DIS at HERA  using 1996 data • DIS data sample compared to pQCD + parton showers + hadronization models predictions • Event kinematics and tracking well understood and simulated • Plan • Increase statistics of data and simulation events • include all 96 and 97 data • Investigate diffractive effects • Measure multiplicities vs. Q^2 in the current and target region of the Breit frame • Measure momentum spectra • Compare different models for parton showers and hadronization to data • Evaluate systematic uncertainties

40. Diffraction & max Diagram Event Display DIS Diffraction • 5-10% of events • Not modeled by • Ariadne Diffractive events expected at large angles (low max)

41. Look for Diffractive Events • Only max>3.2 is described by Ariadne • ~500 events out of total data sample (4798 events) • These are diffractive events that are not simulated by the Ariadne Monte Carlo Note: Log Scale • Solutions: • (fast) Cut out diffractive events • (better) Use mixture of diffractive & non-diffractive monte carlo. • will do both Normalized above max= 3.2

42. RapGap + Ariadne 10<Q2<1200 # of events # of events Data & fitted MC mixture 16% RAPGAP+ 84% Ariadne Mix Diffractive & Non-Diffractive Models • RapGap contains diffraction; Ariadne does not • A fit to a superposition of RapGap and Ariadne max distributions, determines relative contribution of each

43.   lepton    photon S S S  gluon quark A = A0 + A1aS + A2aS2 +... Leading Order (LO) Next to Leading Order (NLO) pQCD series Perturbative QCD (pQCD) series: